fix(input): do no reinterpret mouse keys with ALT modifiers

Remove check for MOD_MASK_META as it is for <T- which never appears in TUI.
Make small changes to docs.

runtime/doc/cmdline.txt

@@ -240,7 +240,7 @@ CTRL-[						*c_CTRL-[* *c_<Esc>* *c_Esc*
 		Note: If your <Esc> key is hard to hit on your keyboard, train
 		yourself to use CTRL-[.
 						*c_META* *c_ALT*
-		ALT (|META|) acts like <Esc> if the chord is not mapped.
+		ALT (|META|) may act like <Esc> if the chord is not mapped.
 		For example <A-x> acts like <Esc>x if <A-x> does not have a
 		command-line mode mapping.
 							*c_CTRL-C*

runtime/doc/insert.txt

@@ -39,7 +39,7 @@ char		action	~
 		abbreviation.
 		Note: If your <Esc> key is hard to hit, try CTRL-[ instead.
 						*i_META* *i_ALT*
-		ALT (|META|) acts like <Esc> if the chord is not mapped.
+		ALT (|META|) may act like <Esc> if the chord is not mapped.
 		For example <A-x> acts like <Esc>x if <A-x> does not have an
 		insert-mode mapping.
 						*i_CTRL-C*

runtime/doc/intro.txt

@@ -383,8 +383,8 @@ Note:
   <k1>, ..., <k9> and <kPoint> will not work.
 - Nvim supports mapping multibyte chars with modifiers such as `<M-ä>`. Which
   combinations actually work depends on the the UI or host terminal.
-- When a key is pressed using a meta or alt modifier and no mapping exists
-  for that keypress, Nvim behaves as though <Esc> was pressed before the key.
+- When a key is pressed using a meta or alt modifier and no mapping exists for
+  that keypress, Nvim may behave as though <Esc> was pressed before the key.
 - It is possible to notate combined modifiers (e.g. <C-A-T> for CTRL-ALT-T),
   but your terminal must encode the input for that to work. |tui-input|
 

runtime/doc/vim_diff.txt

@@ -250,7 +250,7 @@ Input/Mappings:
   <M-1>, <M-BS>, <M-Del>, <M-Ins>, <M-/>, <M-\>, <M-Space>, <M-Enter>, etc.
   Case-sensitive: <M-a> and <M-A> are two different keycodes.
 
-  ALT behaves like <Esc> if not mapped. |i_ALT| |v_ALT| |c_ALT|
+  ALT may behave like <Esc> if not mapped. |i_ALT| |v_ALT| |c_ALT|
 
 Normal commands:
   |gO| shows a filetype-defined "outline" of the current buffer.

runtime/doc/visual.txt

@@ -161,9 +161,10 @@ If you want to highlight exactly the same area as the last time, you can use
 							*v_<Esc>*
 <Esc>			In Visual mode: Stop Visual mode.
 						*v_META* *v_ALT*
-		ALT (|META|) acts like <Esc> if the chord is not mapped.
+		ALT (|META|) may act like <Esc> if the chord is not mapped.
 		For example <A-x> acts like <Esc>x if <A-x> does not have a
 		visual-mode mapping.
+
 							*v_CTRL-C*
 CTRL-C			In Visual mode: Stop Visual mode.  When insert mode is
 			pending (the mode message shows